1. Field of the Invention
The present invention relates to a servo motor control device which can execute a feedback control for a servo motor, and more particularly, to a servo motor control device which can detect a rotational position of the servo motor, that is, a specific phase of the servo motor.
2. Description of Related Art
Conventionally, there is a control device which detects slits by a photosensor in order to control a rotational speed of an electronic servo motor and executes a feedback control for the servo motor based on an encoder signal from the sensor, wherein an encoder disk plate having a plurality of slits is fixed to a servo motor driving shaft.
In order to detect a reference phase which is used for determining an initial setting position of the servo motor while executing the feedback control for the servo motor, a phase setting slit is formed independently of a rotational speed detecting slit in one location of the encoder disk plate. Further, a sensor is formed independently of a rotational speed detecting sensor in order to detect the phase setting slit.
As described above, a problem is encountered in that two sensors are needed so that the control device can detect the reference phase of the servo motor while executing the feedback control for the servo motor. One sensor is used for the feedback control which controls the rotational speed of the servo motor. The other sensor is used for detecting the reference phase. Therefore, one phase setting portion is formed by enlarging the interval between two specific slits such that only one sensor is used for both the feedback control and for detecting the phase setting portion. That is, only one sensor detects the slits. Thus, the control device can execute the feedback control based on the signal cycle of the encoder signal from the sensor. Simultaneously, the control device can detect the phase setting portion because the encoder signal is longer than the normal encoder signal corresponding to the predetermined interval when the sensor detects the phase setting portion.
However, the above-mentioned control device has the following two problems. When the sensor detects the phase setting portion, the encoder signal cycle from the sensor is longer than the normal encoder signal cycle. The rotational speed of the disk plate then increases because the control device keeps executing the feedback control based on the encoder signal from the sensor in the detecting operation of the phase setting portion. Thus, a speed control has an accidental error. Moreover, since the rotational speed of the servo motor increases, the slit interval time in the phase setting portion does not become longer than the slit interval time in the rotational speed detection. Therefore, the control device may execute an error detection of the phase setting portion.